Feasibility experiment for Active Monitoring of Inter-plate Coupling in Tokai region. ---A dense array measurement---

We carried out a long-distance seismic monitoring experiment using ACROSS (Accurately Controlled and Routinely Operated Signal System) for 10 months starting from the end of 2004 in Tokai region central Japan. In this experiment, we attempted to detect reflected phases from the top surface of the subducting Philippine Sea plate and to detect their temporal changes. In the Tokai region, a seismic survey was conducted in 2001. A strong reflected phase was detected and was interpreted as a reflection from the boundary between subducting and overriding plate [Iidaka, 2003]. Yoshida et al. [2004] analyzed the ACROSS signal received by a nationwide seismic network (Hi-net) and identified several phases which may include the direct waves and reflected phases from the plate boundary. The aim of our research is to confirm the reflected phases and also to detect temporal changes in properties of them. The ACROSS source was continuously operated in Toki City, Gifu prefecture by Tono Geoscience Center. The frequency-modulated signal with frequency band from10 to 20 Hz was precisely repeated with an interval of 50 seconds. As the rotation direction of the source reverses once per hour, we can synthesize linear vibration in any direction. Seismometers were deployed on a survey line between 40 and 70 km distance to the southeast from the source. We also deployed an array consisted by 12 seismometers having 2km aperture at 55 km away from the source on the survey line. We acquired seismograms with the array and stacked to improve S/N ratio. In ACROSS data analysis, we can estimate the errors of the received signals quantitatively in the frequency domain. We stacked the received signals weighted by inverse of the estimated errors in order to reduce the effect of the incidental noises such as earthquakes. We converted the stacked spectral signals into a frequency response by dividing them by the source spectra. Through applying an inverse Fourier transform to the frequency response, we obtained the transfer function in the time domain, in which P, S and some later phases were identified. We applied an array analysis to these transfer functions to identify the later phases which are coherent among the array elements. We calculated the semblance coefficients for the phases arriving after the direct P waves. We searched the combination of the slowness and the incident angle that gives the highest semblance coefficient. As a preliminary result, though we found a large trade-off between the slowness and the incident angle, we also found some coherent phases arriving to the array after direct P phase. Comparing the travel-time with synthetic waveform, we conclude that these phases may include reflected waves from the top of subducting Philippine Sea plate, Moho discontinuity or some other layer boundaries. The data analysis is underway and will be presented in detail in the conference.